In the method for manufacturing a seamless pipe, a “billet” as a raw material is heated in a heating furnace, and then it is made into a hollow shell by piercing-rolling using a piercer. The hollow shell is processed to adjust its pipe wall thickness by mandrel mill rolling. Thereafter, for example, outer working of the hollow shell is done by an extractor sizer to form a finished pipe product.
In the mandrel mill rolling, a mandrel bar is inserted into a hollow shell which has been treated by piercing-rolling and is set at a high temperature within the range between 1000 and 1300° C., and then, drawing-rolling of the hollow shell is carried out, for instance, by a pair of caliber rolls set at 90 degree of phase difference in seven to eight tandemly-placed mandrel mills. When drawing-rolling the hollow shell, relative slippage happens between the inner surface of the hollow shell and outer surface of the mandrel bar. So, in the mandrel mill rolling, so as the relative slippage to be smoothly developed, lubrication in the working interface has to be favorably secured. Accordingly, by applying a lubricant in the working interface and securing low and stable friction coefficient, seizure between the hollow shell and the mandrel bar is inhibited so that favorable inner-surface quality and dimensional accuracy of the finished pipe product can be obtained.
There are mainly two examples of method for lubricating working interface, i.e., a method by adhering a lubricant onto the surface of mandrel bar; and the other method by adhering a lubricant to the inner surface of the hollow shell. As the method by adhering a lubricant onto the surface of mandrel bar, a method having the steps of: applying a water dispersive lubricant mainly containing black lead and a resin-series organic binder to the surface of mandrel bar before inserting it into the hollow shell; and drying the water dispersive lubricant to form a solid lubricating coating on the surface of mandrel bar.
On the other hand, as a lubricant to be used for adhering onto the inner surface of the hollow shell, hitherto, various lubricants have been studied; however, none of them shows sufficient effects. Particularly, technology about mandrel mill rolling has been developed and retained-mandrel mill rolling for keeping the speed of bar under rolling constant has been spread; however, in the retained-mandrel mill rolling, load to the mandrel bar is large so that conventional lubricant cannot meet the required lubricity and inner-surface quality of the finished pipe product.
Further, in a full-retractable mandrel mill which becomes popular these years, a short mandrel bar is used for drawing-rolling a long pipe product. So, reduction of friction coefficient becomes necessary and load to a mandrel bar becomes larger with the conventional lubricant; whereby seizure tends to occur, which deteriorates the inner-surface quality of the pipe product.
Because of this, in order to reduce friction between hollow shell and mandrel bar by making properties of the lubricant in powder state favorable and making the lubricant evenly applicable to the predetermined area when applied onto the inner surface of the hollow shell, Patent Document 1 proposes a powder lubricant composition for hot metal working which contains sodium borate pentahydrate as a main component and sodium carbonate and the like as a supplemental lubricant.
When mandrel mill rolling is carried out using the powder lubricant composition for hot metal working described in Patent Document 1, as the powder lubricant composition is not only excellent in handleability and workability but also able to reduce friction between the hollow shell and the mandrel bar at a time of rolling, it is possible to reduce flaws produced on the inner surface of the finished pipe product.
Chromium-plated surface of the mandrel bar is usually covered with chromium oxide, namely, surface of the chromium-plate becomes passivity so that the mandrel bar is in a state hardly corrosive. But, when surface of the mandrel bar and a substance like sodium borate which melts metal oxide are contacted each other at high temperature, chromium oxide in the surface of the chromium-plate is melted and a kind of corrosive wear is sometimes caused.
In order to solve the problem, Patent Document 2 proposes a powder lubricant composition for hot metal working containing sodium borate, which is capable of inhibiting corrosive wear of chromium layer on the surface of the mandrel bar but also capable of prolonging the life of tools. When mandrel mill rolling is carried out by using the powder lubricant, it is possible to inhibit corrosive wear of chromium-plate surface and to prolong the life of hot-metal-working tools; it is also possible to favorably and stably maintain the inner-surface quality of the finished pipe product.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-338984    Patent Document 2: JP-A No. 2002-338985